1. Field of the Invention
The present invention relates generally to a control system for an automatic power transmission for an automotive vehicle. More specifically, the invention relates to an electronic control system for an automotive automatic power transmission, which performs in a fail-safe mode of operation in response to failure of a throttle angle sensor.
2. Description of the Background Art
Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 63-62656 discloses a control system for an automatic power transmission for an automotive vehicle. The disclosed control system is responsive to failure of a throttle angle sensor to perform in a fail-safe mode of operation, in which a line pressure is controlled to a predetermined position irrespective of an actual position of a throttle valve. In the shown system, the fail-safe mode of operation is performed by setting the line pressure at a maximum value corresponding to a throttle valve fully open position. By setting the line pressure at the maximum value, it becomes possible to maintain engagement of the friction elements, such as clutches for enabling the vehicle to travel.
In the modern electronically controlled automatic power transmission, a microprocessor-based control unit performs not only line pressure control but also shift control, and lock-up control, clutch control for effecting engine braking. These controls are normally performed based on a plurality of preselected transmission control parameters, including a throttle valve open angle monitored by the throttle angle sensor. For such type of automatic power transmission, failure of the throttle angle sensor may affect not only line pressure control but also other controls, e.g. shift control, lock-up control, engine brake clutch control and so forth. Assuming that a common back-up signal as a replacement of the throttle angle signal from the throttle angle sensor is used, the back-up signal may represent the throttle valve fully open position in order to assure engagement of the clutches to drive the driving wheels. If such back-up signal is applied for shift control, it serves as a demand for swift acceleration. Therefore, the shifting pattern for shifting transmission speed ratio is set in a kick-down range pattern to set a shift-up point at a higher engine speed than that in the normal shift pattern. On the other hand, such back-up signal serves to establish a lock-up state of a lock-up clutch. On the other hand, since the back-up signal represents a full load condition on the engine, the clutch for effecting engine braking is maintained at a release state.
Such status of components of the automatic power transmission may degrade drivability of the vehicle.